This relates generally to dual activity drilling from a drilling ship.
Generally, when drilling in deep water environments, drilling mud is forced down from a drilling ship into a subsurface formation. As used herein, the term “drilling ship” encompasses a floating platform capable of propulsion on its own or by being towed, pushed or pulled, and includes semi-submersible and self-propelled vessels.
When the drilling mud pressure is high, the possibility of fracture and leakage of the formation increases. When the drilling mud pressure is low, the possibility of blowout when the drilling mud pressure is less than the pore pressure arises. Generally, the mud pressure increases with depth. Thus, the deeper the formation, the more prone the formation is to fracture and the more shallower portions of the formation may be more prone to blowout. Thus, the pore pressure is higher the deeper the borehole goes. This means that mud pressure must be increased for well control. In such case, it is necessary to isolate that higher mud pressure from the shallower portions of the formation using casings.
With depth, the pore pressure in the rock and the fracture pressure in the rock begin to diverge. The physics of the subsurface makes it impossible to drill a hole through this transition zone as increased equivalent circulating density through friction of returning drilling mud and the open hole limits the depth the hole can be drilled before exceeding the fracture pressure of the rock. Casing, therefore, is set and cemented.
Therefore, in subsurface situations where there are drilling hazards, such as shallow water flow, it is desirable to drill the top holes using the “pump and dump” drilling method and to set and cement the casing at a depth where drilling can be formed with an equivalent circulating density less than the fracture pressure.
Often, several strings of casing are necessary, including a 36 inch conductor, a 30 inch casing, and a 24 inch casing, which are set and cemented before the 20 inch casing is set, enabling the subsurface blowout preventer and marine riser to be installed on the wellhead.
With the pump and dump drilling technology, the drilling mud is water based and environmentally acceptable to dump on the seabed. The drilling mud needs to have the appropriate rheological properties to assure a stable well bore is maintained. In deep water drilling areas, like the Gulf of Mexico, it is not uncommon to use and lose up to 30 to 40 thousand barrels of mud while drilling these top holes. This may create logistical problems replenishing mud stocks on the rig.